Monday's CME Group slide in grain prices continued overnight and looks to at least start Tuesday's session on the same note, sources say. Early calls are corn 4-6 cents lower, soybeans 8-10 lower and wheat 5-7 lower, according to private sources.

As of 6:40 a.m. CDT, May corn futures were 7 1/4 cents lower at $6.56 1/4 per bushel, according to Barchart.com. May soybeans were 10 1/2 cents lower at $13.56 per bushel, while May wheat was 7 3/4 lower at $6.44 1/2.

• See more of the latest commodity prices

On Monday, the grain markets divorced themselves from largely bullish outside markets, focusing instead on supply issues and looking ahead to the March 30 USDA Prospective Plantings report 10 days from now.

Meanwhile, with unseasonably warm temperatures, farmers are turning their attention to planting and any remaining fieldwork.

• Talk: When are we officially 'planting?'
• Also: Corn planted in Iowa
• Read more: Is it 'nuts' to plant now?